You might unplug your kickdown switch under the go pedal, and take if for a drive and see if that solves the issue.

If you are shifting into other gears at higher RPM, You might have a Bowden Cable issue. I don't know if the SD's have bowden cable but if so, maybe it's adjusted too high and the rpms aren't getting high enough to shift into 4th. Try adjusting it down and see what happens. I don't remeber off hand if it needs to be adjusted CW or CCW to make shifts sooner. You'll have to look it up.

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